Re: msync() behaviour broken for MS_ASYNC, revert patch?

[Trond Myklebust]

On Fri, 2006-02-10 at 14:46 -0800, Linus Torvalds wrote:
> 
> On Fri, 10 Feb 2006, Trond Myklebust wrote:
> > 
> > The Single Unix Spec appears to have a very different interpretation.
> 
> Hmm. Very different wording, but same meaning, I think.
> 
> >         When MS_INVALIDATE is specified, msync() shall invalidate all
> >         cached copies of mapped data that are inconsistent with the
> >         permanent storage locations such that subsequent references
> >         shall obtain data that was consistent with the permanent storage
> >         locations sometime between the call to msync() and the first
> >         subsequent memory reference to the data.
> 
> Again, this says that the _mapping_ is invalidated, and should match 
> persistent storage.
> 
> Any dirty bits in the mapping (ie anything that hasn't been msync'ed) 
> should be made persistent with permanent storage. Again, that is entirely 
> consistent with just throwing the mmap'ed page away (dirty state and all) 
> in a non-coherent environment.
> 
> I don't think we really have any modern Unixes with non-coherent mmap's 
> (although HP-UX used to be that way for a _loong_ time). But in the 
> timeframe that was written, it was probably still an issue.
> 
> Now, in a _coherent_ environment (like Linux) it should probably be a 
> no-op, since the mapping is always consistent with storage (where 
> "storage" doesn't actyally mean "disk", but the virtual file underneath 
> the mapping).

Hmmm.... When talking about syncing to _permanent_ storage one usually
is talking about what is actually on the disk. In any case, we do have
non-coherent mmapped environments in Linux (need I mention NFS,
CIFS, ... ;-)?).

IIRC msync(MS_INVALIDATE) on Solaris was actually often used by some
applications to resync the client page cache to the server when using
odd locking schemes, so I believe this interpretation is a valid one.

Cheers,
  Trond

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